This invention relates to an alumina-based ceramic composition suitable for use in the field of electric communication such as microwave dielectric resonators, microwave-guide circuits or lines, microwave capacitor, microwave integrated circuit substrate, and so on; or in the field of high energy physics such as microwave transmitting window for nuclear fusion apparatus, and so forth.
In recent years, with development in the communication networks, the range of frequency used has been expanded to that of the microwaves. In connection with this, dielectric ceramics are widely employed in the region of the microwave frequencies, for impedance matching in various microwave circuits, beginning with the dielectric resonators, and others.
Most recently, a large number of dielectric ceramic elements are required as filters, guns, or frequency stabilizers for FET microwave oscilators and the need is still increasing. The size of the microwave circuit is determined based on the wavelength of the electromagnetic wave. Its wavelength is defined as .lambda.o/.epsilon. where .lambda.o is the wave length in vacuum and .epsilon. is specific dielectric constant when the microwave is transmitted within the three-dimensional microwave circuit made of a dilectric material. On the other hand alumina ceramics have been known to have a high mechanical strength and a low dielectric loss, and to be of a low cost.
Therefore, as this kind of the dielectric ceramics, much attention has been drawn to alumina ceramics.